The long-term goal of the proposed study is to understand the disease-relevant toxic properties of the - amyloid peptide (A), which is strongly implicated in the etiology of Alzheimer's disease (AD). The central hypothesis of this proposal is that the ability of A to damage membranes contributes to Alzheimer's disease pathology, and tau hyperphosphorylation, a hallmark of AD pathology, is a consequence of a neuronal response to membrane damage. We propose that neuronal cell death in AD is a result of chronic tau phosphorylation resulting from chronic membrane damage caused by oligomeric forms of A. This hypothesis is supported by our demonstration that single residue substitutions in A that block its ability to permeabilize membranes also render this peptide non-toxic in a variety of in vitro and in vivo models. Furthermore, preliminary studies reveal that primary neurons exposed to the membrane pore- forming toxin streptolysin O (SLO) show patterns of tau hyperphosphorylation very similar to that induced by exposure to A. We will test this hypothesis using primary neuronal cultures and a novel C. elegans model that enables us to visualize A-induced membrane repair in living animals. This model will allow us to better define the toxic A species by engineering informative A variants (including known familial AD mutations) and assaying their ability to induce membrane repair. Critically, this model will also allow us to genetically test the disease relevance of the membrane damage model by mutating worm orthologs of AD risk genes potentially involved in membrane repair (e.g., PICALM, BIN1, and CTNNA2), and determining if this alters A-induced membrane repair. These studies will be complemented by using primary hippocampal neurons to confirm tau phosphorylation as a component of membrane repair, and to determine where along the repair pathway this event occurs. Relevance: Identification of compounds that block A toxicity (rather than A accumulation) has been hindered by uncertainty regarding the toxic A species and its mechanism of action. The deposition of insoluble, hyperphosphorylated tau in AD is believed to be a downstream consequence of A accumulation, but the biological rationale for why this occurs has not been established. Our proposed studies can potentially provide new leads for the development of AD therapeutics, as well as explain the altered tau metabolism observed in a range of tauopathies in addition to AD.